The invention relates to a process and apparatus for the gas carburizing of steel, wherein the steel part is exposed within a carbon-enriched gas atmosphere of a furnace or the like. The invention also relates to a diffusion process for the formation of a case with an increased carbon content determined as a function of the distance from the surface and wherein in certain time intervals the values important for the diffusion process, including temperature, are determined and used as control parameters to effect the diffusion process.
Gas carburizing processes, such as those used for the case hardening of steel parts, are known, for example in "Zeitschrift fur wirtschaftliche Fertigung", No. 9, September, 1968, pages 456 to 464. During the carburization of a steel workpiece, its surface layer is enriched in carbon. The objective is to obtain a certain gradient of the carbon contained by the completion of the process, with the target values of "depth of carburization", "case carbon content" and "variation of C in the case". This process may be calculated on the basis of the laws of diffusion, as in addition to the temperature and time, certain so-called boundary conditions enter the process as variables, whereby the availability of carbon on the surface is defined.
While in the early days of the technology of case hardening it was assumed that the C content of the surface remains approximately constant during the entire process and that, therefore, the depth of carburization is growing with the square root of time, a demand for increasing accuracy dictated that the process parameters of "carbon potential" and "reaction rate" at the surface be introduced. Depending on the values being measured, the mathematical relationships and the other process values, the variation of C may thus be precalculated, with the carburization of the boundary layer being based on the control of the C potential of the gas atmospehre in the furnace chamber, so that the desired target values may be obtained. It is further known by German DE-P No. 31 39 622 to vary the C potential during carburization with the aid of a computer unit in order to prevent the formation of carbides at the surface of the workpiece in the shortest possible time.
This manner of controlling carburization processes is feasible, however, only if so-called equilibrium atmospheres are present, as only then is the C potential defined and measurable. Atmospheres supersaturated with hydrocarbons have usually been heretofore avoided in view of their limited controllability, even though they could provide an even greater availability of carbon, leading to savings in raw material and energy.
It is, therefore, an object of the present invention to develop a process of the afore-mentioned type, whereby also atmospheres supersaturated with hydrocarbons may be used in carburizing processes, in a manner such that it is possible to unambiguously control and determine the variation of carbon in the boundary layer of the workpiece.